I think what she is trying to say is when the mail carrier has a package for you, he/she puts a key to the big locked mailbox containing your package, in your mailbox so you can open the box. Sounds like the mail carrier put the key in the wrong box, but that neighbor was honest and left the package alone instead of stealing it.
Our old neighborhood had mailboxes like that.
We solve that problem by having all packages from mail order sent to our offices instead of home. 4951